coverity: avoid dereference after null check
coverity: avoid dereference after null check

All decompress functions used after this check take into account that
encoder->palette is not NULL. So, if we already detected that the
palette is NULL, let's just return early.

Signed-off-by: Fabiano Fidêncio <2931bae082ef9bc04523ba759c03a4778316d48f@redhat.com>
Acked-by: Christophe Fergeau <05b95054a663c388a4e33d95828cb30f0370f8ef@redhat.com>
Acked-by: Victor Toso <39b7599ff73bbf5db531748ba5250f21fa7120ff@redhat.com>

fgouget/spice-common,fgouget/spice-common,fgouget/spice-common,fgouget/spice-common